Field of the Invention
The present invention relates to suppressing flow-induced vibration (FIV) in a boiling water reactor (BWR) jet pump, and more particularly, to methods and apparatus for determining if a BWR jet pump is susceptible to FIV, and methods and apparatus for suppressing same.
Description of Related Art
The methods and apparatus described herein in connection with the present inventions are particularly adapted for a widely used boiling water reactor design of General Electric Co. FIG. 1 is a cut-away isometric depiction of a typical such conventional BWR taken from GE U.S. Pat. No. 6,434,208. Although FIG. 1 is highly schematic, it is nevertheless sufficient to afford an understanding of the construction and application of the inventive methods and apparatus described and claimed herein. Those skilled in the art will understand, however, that these methods and apparatus are not limited to use with or in connection with any particular reactor design.
A conventional BWR such as that shown in FIG. 1 includes a reactor pressure vessel (RPV) 10 supported by a foundation 12. The RPV 10 has a cylindrical side wall 14 and an integral bottom head 16. The side wall 14 terminates at its top end in a mounting flange 18 to which is secured a flange 20 of an RPV top head 22. The flanges 18 and 20 are secured by bolts (not shown) spaced around their circumference. This permits removal of the top head 22 for accessing the interior of the RPV, such as during shutdowns for maintenance or refueling.
A reactor core 24 inside the RPV 10 comprises fuel bundles 26 (some of which are omitted for clarity) of fissionable material. The amount of nuclear fission, and thus the amount of heat generated by the reactor core, is determined by the vertical positions of control rods 28, which are adjusted by control rod drives 30 external to the RPV 10. The core is immersed in water contained in the RPV, and nuclear fission of the material in the fuel bundles 26 produces sufficient heat to convert the liquid water into steam. Steam separators 32 extract water from the steam, and this water is taken from the RPV 10 at ports 34 (only one of which is shown) and led to recirculation pumps (not shown). The steam then passes through a steam dryer 36 to remove any residual water, which is taken from the RPV 10 at ports 38 (only two of which are shown).
The recirculation pumps return water to the RPV 10 via jet pumps, described in more detail below, inside the RPV 10. A core shroud 50 disposed coaxially with the RPV side wall 14 forms an annular region 52 in which a plurality of jet pump assemblies JPA (only one being shown in FIG. 1) are arrayed circumferentially around the core shroud 50 within the annular region 52. Typically, there are ten jet pump assemblies each having two jet pumps, as described in more detail below in connection with FIG. 2. The jet pumps have outlets that mate with jet pump exit openings 54 in a jet pump baffle plate 56 secured to the core shroud 50. In a conventional manner, the let pumps take recirculated water from the recirculation pumps, entrain water from within the annular region as discussed below, and direct the output through the openings 54 for circulation upward through the reactor core 24 to be converted to steam as discussed above.
FIG. 2 is an isometric view of a typical jet pump assembly JPA used in the BWR shown in FIG. 1. The depiction in FIG. 2 (taken from GE Pub. No. US 2012/0219103) is also schematic, but includes sufficient detail for purposes of the present description. One main component of the jet pump assembly is a riser pipe 60 that has a riser inlet elbow 62 providing an inlet 64 to which water from the recirculation pumps is introduced. Typically, there are two recirculation pumps, and half of the riser inlets 64 are attached to a manifold that receives recirculated water from one of them, and the other half of the inlets are attached to a second manifold that receives recirculated water from the other one, although other numbers and arrangements of manifolds and recirculation pumps may be used in different reactor constructions. The riser pipe 60 extends upward from the inlet elbow 62 to a transition piece 66. The riser pipe is secured in place in the annular region 52 of the RPV 10 by welding the inlet elbow 64 to a water inlet nozzle (not shown) at the RPV side wall 14 and to a riser brace (not shown in FIG. 2) secured to the interior of the RPV side wall at a location proximate to the transition piece 66. (The riser brace is depicted schematically in FIG. 5.) Each jet pump assembly includes two essentially identical jet pumps JP.
FIGS. 2 and 3 taken together show constructional details of the jet pumps JP. FIG. 3 is an enlarged view of the region at the top of the jet pump assembly JPA and illustrates details of a transition assembly TA that distributes flow from the riser pipe 60 to the jet pumps JP. The transition assembly TA includes the transition piece 66, which divides the recirculating water in the riser pipe and introduces equal amounts to a pair of mixer elbows 68, each of which includes a lifting eye 69 for a purpose described below. Each jet pump JP includes a mixer 70 with an Inlet section 72 that is rigidly secured to its associated mixer elbow 68 in a suitable fashion, as by welding. The recirculating flow entering the inlet section 72 its associated mixer elbow 68 is fed to a suction inlet 74, where the mixer inlet section 72 terminates in a nozzle 75 that injects a high velocity stream of recirculating water into a mixer throat 76. This high velocity stream entrains the water in the annular region 52 of the RPV 10 (see FIG. 1), and the recirculating flow and the entrained flow enter an elongated mixing chamber 78 that leads from the suction inlet 74 to an end region 79 (See FIG. 4). The design and construction of the suction inlet and mixer throat can vary from installation to installation and the configurations in FIGS. 2 and 3 are slightly different because FIG. 3 is taken from GE U.S. Pat. No. 4,499,691, while FIG. 2 is based on Pub. No. US 2012/0219103. However, the principles of operation are the same and illustrate the point that the present invention as described and claimed herein is applicable to any jet pump design subject to the problems discussed herein. The mixing chamber 78 serves to eliminate any local flow variations in the combined recirculating and entrained flows entering the throat section 76.
As best seen in FIG. 4, the mixing chamber 78 has an exit at an end region 79 that directs the mixed flow into a separate jet pump diffuser 80, which has an increasing cross-sectional area in the downward flow direction to reduce the flow velocity. The flow in the diffuser 80 exits at its bottom end, which is welded to the baffle plate 56, through one of the jet pump exit openings 54. As best seen in FIG. 4, taken from GE Pub. No. US 2008/0029969, the end region 79 at the exit of the mixing chamber fits within the top region 81 at the inlet of the diffuser 80 to form a slip joint 82. Typically, the end region 79 of the mixing chamber is about seven to nine inches in diameter and the diametrical clearance between the inside of the top region 81 of the diffuser 80 and the end region 79 of the mixing chamber is on the order of 0.020 in. The lifting eye 69 on the mixer elbow 68 has a generally circular lifting opening 69a by which the elbow/mixer assembly is lowered into (and extracted from) the annular region 52, and beveled edges on guide ears 84 at the top of the diffuser 80 assist in inserting the end region of the mixing chamber 78 into the close-clearance top region 81 of the diffuser section 80. The slip joint 82 permits relative movement between the mixer 70 and the diffuser 80, as discussed just below.
Returning to FIG. 3, the jet pump assembly JPA further includes a jet pump beam assembly BA that clamps the mixer elbows 68 to the transition piece 66 to secure in place the mixers 70, which as already noted are rigidly attached to the corresponding mixer elbows 68. The construction of the beam assembly can vary from installation to installation, but FIG. 3 illustrates features of BWR beam assemblies particularly adapted for use with the methods and apparatus described and claimed herein. The beam assembly shown in FIG. 3 includes an inner bracket 90 rigidly secured to the transition piece 66 at the side facing the core shroud 50 and an outer bracket 92 rigidly secured to the transition piece 66 at the side facing the RPV side wall 14. (The outer bracket 92 is partially cut away in FIG. 3 to show the top of its associated mixer elbow 68.) Each jet pump JP has an associated jet pump beam 93 that spans the space between the inner and outer brackets above the mixer elbow 68. The ends of the jet pump beam fit into notches 94 in the respective brackets 90 and 92. A beam bolt 96 threads into an opening located centrally of the jet pump beam 93 and into a blind receiving hole 98 in the top of the associated mixer elbow 68. (The jet pump beam at the partially cut away location is omitted to show the receiving hole 98.) The mixer 60 and its attached mixer elbow 68 are secured in place on the transition piece 66 by tightening the beam bolt 96 in the threaded pump beam opening and into the blind hole 98 to press the mixer elbow 68 against the transition piece 66. In some cases, the beam bolt end is a convex spherical section that mates with an insert in the blind hole 98 having a complimentary concave spherical section to assist in properly aligning the beam 93 and the transition piece 66 as the bolt 96 is tightened into place.
The diametrical clearance shown in FIG. 4 between the end region 79 of the mixing chamber 78 and the top region 81 of the diffuser 80 permits these parts to move radially relative to each other. As noted, the slip joint 82 is necessary to allow relative movement of the mixer and diffuser because of factors such as differential thermal expansion between RPV parts typically made of carbon steel with a stainless steel cladding, and jet pump parts generally made of stainless steel or INCONEL® alloy 600, which operate at temperatures of about 550° F. However, under most operating conditions, some of the main flow MF passes through the slip joint 82 as leakage flow LF (which can be in the direction shown by the arrow LF or in the opposite direction into the diffuser), and the resulting complex flow pattern and pressure fluctuations can cause the mixer and diffuser to oscillate in a phenomenon known as flow-induced vibration (FIV). This may lead to increased metal fatigue and premature failure of the jet pump. Performing repairs on BWR jet pumps is a major undertaking, because any repair that requires access to the interior of the RPV represents a significant expenditure and necessitates shutting down the reactor with a concomitant loss of operating revenue. Moreover, the Integrity of the jet pumps must be assured for safety reasons since they maintain the water level in the RPV in the event of a loss of coolant accident. FIV can also cause reactor parts to loosen and create safety issues, and can cause reactor parts to abrade and shed small particles that can contaminate the reactor.
Nevertheless, the mixing chamber 78 and diffuser 80 cannot be clamped together in any manner that would completely prevent their relative axial movement caused by differential thermal expansion. Accordingly, various constructional approaches have been used in attempts to prevent or inhibit FIV and/or the resulting operational problems it causes. A principal constructional approach uses a restrainer bracket RB (see FIG. 2), described in more detail below. This bracket is rigidly mounted to the riser pipe 60 (typically by welding) and has an opening through which the mixing chamber 78 passes. As described in more detail in connection with FIG. 5, the restrainer bracket provides a three-point mount that includes two fixed set screws and a gravity wedge, typically spaced equally around the inner circumference of the ring opening, that bear against the mixing chamber. Theoretically, the gravity wedge is heavy enough to be held against the mixing chamber by gravity and thereby suppress FIV. GE U.S. Pat. No. 6,788,756 discusses more details of the construction and operation of this type of three-point mount, and some of its shortcomings as an FIV-suppressing mechanism.
While prior art structural arrangements intended to alleviate FIV in nuclear reactor jet pumps are effective to one degree or another, they all suffer from shortcomings. Some employ complex arrangements to connect the mixer and diffuser together in one fashion or another at the slip joint while still allowing for their relative axial movement. Examples are described in U.S. Pat. No. 6,394,765, U.S. Pat. No. 6,450,774, and U.S. Pat. No. 8,197,225. Others seek to attack the problem at its source by altering the flow pattern through the mixing chamber/diffuser interface. Examples of this approach are described in U.S. Pat. No. 4,285,770 and U.S. Pat. No. 6,438,192, and in Pub. No. US 2008/0029969, Pub. No. US 2008/0031741, and Pub. No. US 2012/0219103.
Prior approaches to alleviating FIV that involve the introduction of additional parts at the region of the slip joint add expense (both in materials and installation cost), increase the likelihood of loose parts in the RPV, and in some cases can increase the difficulty of removing the mixer for replacement or repair. U.S. Pat. No. 6,788,756 discusses modifications to the gravity wedge arrangement discussed just above, with the object of rendering it more effective in suppressing FIV, but those modifications also require additional parts, are difficult and costly to implement, and are not always effective. Generally speaking, any approach that uses additional hardware complicates inspection of the jet pump assembly, reduces clearances and thus makes access for maintenance to areas in the annular region 52 more difficult, and adds a potential cause of failure. Other prior approaches, particularly those that seek to alter the flow pattern at the slip joint, require machining a special contour on the mixing chamber or the diffuser at or near the slip joint. Machining these parts can be expensive, and the resulting alteration in the flow pattern at the slip joint may not alleviate FIV under all operating conditions. Moreover, this approach requires special handling since a mixer that has already seen service in an operating reactor will be highly radioactive, thus adding even more to the cost and complexity of implementing this approach.
Another complicating factor is that not all jet pumps exhibit flow-induced vibration when the reactor is operating. For example, experience shows that in a typical BWR with 20 jet pumps, all of which are made to the same specifications and for all intents and purposes are identical, it is nevertheless common for only some to experience FIV. In addition, one reactor installation may have certain of its jet pumps experience FIV while another reactor installation will have more or fewer jet pumps that experience FIV and that are in different locations around the reactor core-even though the different reactors and the jet pumps are nominally identical.
Another approach to dealing with jet pump FIV is to apply one of the known FIV-alleviating approaches preemptively to all of the jet pumps in a particular reactor, before even knowing which ones, if any, will experience FIV during reactor operation. This is also an imperfect solution because it “fixes” jet pumps that might not be susceptible to FIV in the first place. And since all of the known approaches are expensive, they add to reactor cost, probably unnecessarily to some extent (since many of the jet pumps likely would not have experienced FIV anyway). Moreover, some prior art approaches can even exacerbate rather than alleviate FIV.